Sergey Chikuyonok has created a really nice, easy to use, and good looking content assist library for input text fields and text areas:

It calculates precise position of characters in <textarea> or <input> tag and places completion proposals popup under it. You can explicitly call popup window to assist/replace word (depends on caret position) with Ctrl+Space (buggy in Firefox and Opera) or Alt+Space (buggy in Opera). The project is based on Eclipse IDE’s content assist architecture, it has some common class names and methods. The default implementation works with fixed words vocabulary, but developers can easily modify or extend some classes to match their needs.